A typical multi-color dye donor web that is used in a dye transfer or thermal printer has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. In addition, there may be a transparent colorless laminating section immediately after the color sections.
Each color section of the dye donor web consists of a dye transfer area which is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are often not used for printing. The dye transfer area may be about 152 mm wide and the two longitudinal edge areas may each be about 5.5 mm wide, so that the total web width is approximately 163 mm.
To make a multi-color image print using a thermal printer, a motorized donor web take-up spool draws a longitudinal portion of the dye donor web off a donor web supply spool in order to successively move an unused single series of yellow, magenta and cyan color sections over a stationary liner array (bead) of selectively heated resistive elements on a thermal print head between the supply and take-up spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred line-by-line, via the selectively heated resistive elements, onto a dye receiver medium such as a paper or transparency sheet or roll, to form the color image print. The selectively heated resistive elements often extend across the entire width of a color section, i.e. across the dye transfer area and the two longitudinal edge areas comprising that color section.
As each color section is drawn over the selectively heated resistive elements, it is subjected to a longitudinal tension particularly by the forward pulling force of the motorized donor web take-up spool. Since, the dye transfer area in the color section is heated by the resistive elements the web is weakened, making the web vulnerable to being longitudinally stretched if too much tension is applied. Consequently, too much longitudinal tension will stretch the donor web in the dye transfer area which in turn causes some creases or wrinkles to develop in the dye transfer area. As the dye donor web is pulled by the motorized donor web take-up spool over the selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing (rear) end portion of a used dye transfer area at least to a leading (front) end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver medium, although they may be relatively short, are quite visible. This indicates that too much tension on the dye donor web will result in creases or wrinkles being created in an unused dye transfer area and line artifacts being printed on the dye receiver medium during the dye transfer process.
More significantly, as each color section is drawn over the selectively heated resistive elements too little tension will cause the web to be slack. Decreasing the tension further will cause more slackness in the web. This will result in improper peeling or delamination of the dye donor web from the receiver web. Improper peeling ranges from the peel position of the web shifting from the desired location at the peel bar to the extreme of not delaminating at all and causing a printer jam. When the peel location of the dye donor web is not at its desired location there is a high probability for defects to occur in the printed image. These defects include spot defects, creases, sticking defects and streaks. Many of these defects are due to the fact that the donor web will selectively stick to the receiver web at specific locations if there is not enough web tension to maintain the peel location. Spot defects are regions of low and high print density caused by micro folds in the donor sheet due to too little tension in the donor web. Sticking defects are due to the detachment of the dye layer in a thermal donor from the PET (polyethylene terephthalate) support and transfer of the dye layer to the receiver during the peel process following printing. This is a serious and unacceptable problem for the customer because it results in high density dye specs being scattered across the face of the receiver.
Thus, there is a need to maintain a desired peel location of a first web from a second web in thermal printers. The first web is usually a donor media (“dye donor web”) containing the colorants that are thermally transferred to the receiver media (second web). The receiver media is usually the final hardcopy print. The transfer process needs to be carefully controlled so that the correct amount of colorant(s) is transferred to produce a high quality image on the receiver material. During the printing process the two webs are brought in contact at the print head where thermal lamination occurs during the dye transfer process. After lamination, the two webs must be separated from each other in a controlled fashion. This separation is achieved by applying a known force to the laminated layers at a fixed location known as the peel location. This force is usually applied by tensioning the webs and forcing the webs to travel in different directions as they pass the desired peel location. The desired peel location is at a peel bar. If the force applied to the webs is insufficient then the webs will not separate at the desired peel location. When this occurs the quality of the print can be adversely affected and in severe cases the webs stay laminated together and cause the printer to jam. If the force applied to the web is too large then the webs may deform and introduce printing artifacts.
Different donor and receiver materials will have different binding forces when thermally laminated together and will therefore require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer. Furthermore changing environmental conditions such as ambient temperature and humidity can also cause the binding forces to change for a given set of donor and receiver webs. Product variability resulting from material variations can also affect the binding forces. All sources of variation in the binding forces between a pair of donor and receiver webs will require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer.
U.S. Pat. No. 6,315,471 by C. Hsieh and C. Chung entitled “Apparatus for Controlling Ribbon Tension in a Thermal Printer” describes an apparatus and method for controlling the tension on the web by pulse width modulation (duty cycle control) by monitoring the input and output diameters of the web on the supply and take-up reels and setting up a transforming table for varying the pulse width modulation as a function of web diameters to keep uniform tension on the ribbons (web).
U.S. Pat. No. 6,082,914 by G. Barrus and K. Moore entitled “Thermal Printer and Drive System for Controlling Print Ribbon Velocity and Tension” describes a thermal printer having a supply of media with a rotatable platen on which the media is moved for printing by a thermal printing head. A supply spindle supplies print ribbon from a supply spool mounted thereon, and a take-up spindle takes up the used print ribbon on a take-up spool. The spindles are each driven by a motor and controlled by a controller which detects the Back EMF (BEMF) of the motors, and calculates the velocity of the spindles, spool, and print ribbon to control each motor based on the BEMF.
Commonly assigned U.S. Pat. No. 6,859,221 by Z. J. Gao, R. F. Mindler and J. F. Corman entitled “Preventing Crease Formation In Donor Web In Dye Transfer Printer That Can Cause Line Artifact On Print” describes a method of preventing crease formation in a dye transfer area of a dye donor web that can cause a line artifact to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver in a dye transfer printer by controlling the heat distribution over the dye transfer area.
Commonly assigned U.S. Pat. No. 6,977,669 by Po-Jen Shih et al. entitled “Preventing Crease Formation in Donor Web in Dye Transfer Printer That Can Cause Line Artifact On Print” describes a thermal printer which employs the method described in commonly assigned U.S. Pat. No. 6,859,221.
U.S. Pat. No. 6,922,205 entitled “Color Thermal Printer And Color Thermal Printing Method” by M. Shusuke describes a thermal printer which conveys a recording sheet at a certain speed by keeping tension applied to a conveyor roller pair within a range designed not to influence the conveyance speed.
None of the prior art can ensure that the location of the peel location is correct and that the thermal printer is working at its designed print resolution. Thus there is a need for a thermal printer that includes a sensor system which can determine the actual peel location of the first web and the second web and to adjust the web tensions so that the peel location will be maintained at the desired peel location within desired tolerance limits for various combinations of thermal web media. Such a thermal printer will tolerate broader variations in manufacturing of the media which affect the tension requirements. Thus, the media may not need to have as tight manufacturing tolerances which would lead to less waste in media manufacturing. This thermal printer will also be able to accommodate for changing environmental conditions which change the peel force and will also result in decreased incidence of machine jams. With such a sensor system installed in a thermal printer, the web tension for new web materials can be automatically adjusted to enable for use in an existing printer.